1. Field of the Invention
The present invention relates to a digital television set, and in particular, to a fine tuning method for an ATSC digital television set.
2. Description of the Background Art
In an analog television set, techniques for a fine tuning apparatus are widely known and make it possible to provide a clean image and clear sound by tracking a transmission frequency distorted at a broadcasting station, a repeater, or a public hearing apparatus, and precisely tuning in to the same.
The above-described fine tuning techniques are roughly divided into two modes. The first mode is an automatic fine tuning (AFT) method, in which a frequency is finely tuned by observing an AFT voltage generated at an intermediate frequency (IF) stage, while tuning in to a predetermined modulation frequency corresponding to a user-selected channel using tuning data outputted from a microprocessor. At this time, the automatic tuning is carried out within a limited range of frequency, not covering a possible channel deviation range (xe2x88x923 MHzxcx9c+3 MHz).
The second mode is a manual fine tuning method, in which a user moves a frequency to the maximum frequency position using a tuning frequency while observing a display state after channel switching.
However, a problem arises in that the fine tuning method used in such an analog television set is not available in a digital television set. The first reason is that it is difficult to generate an AFT voltage in case of fine tuning. In other words, the full-in range of a digital tuner is about one-fourth(approximately 400 KHz) of that of an analog tuner, which is too narrow.
In a case where a level of PLL (Phase Locked Loop) for tuning is at 125 KHz, locking is lost by the movement of the frequency by only one or two levels. Thus, it is almost impossible to generate an AFT voltage. Nevertheless, simplified automatic fine tuning is available because it is determined whether or not tuning is achieved, not by observing an AFT voltage, but by observing whether or not segment synchronization is applied to an analog waveform demodulated into a base band.
However, unlike the analog television set in which a horizontal synchronization is simply sliced, it is checked whether or not synchronization is obtained by observing synchronization patterns uniformly contained in a value that a data symbol can have. Thus, a problem arises in that it takes a large amount of time for tuning.
Secondly, it is impossible to set a frequency to an optimum frequency position while viewing a display state in case of manual fine tuning in the digital television set (unlike the analog television set). This is because of the specific characteristics of the digital television set, in which, when a signal to noise ratio (SNR) is greater than a certain level, an errorless image is displayed though it may be distorted at the maximum frequency position.
Thirdly, since digitally-encoded video signals are transferred being mixed with audio signals and all kinds of additional information, it takes a great deal of time (4xcx9c5 seconds) to parse data and scatter video signals on a screen whenever a frequency moves little by little. Therefore, it is impossible for a user who is accustomed to manual fine tuning in an analog television set to perform manual fine tuning while actually observing a screen.
Accordingly, it is an objective of the present invention to provide a fine tuning method for a digital television set which makes it possible to tune a frequency of a channel delivered from a broadcasting station during channel switching that is deviated from a predetermined frequency or a frequency distorted when it is outputted from an unattended repeater or a public hearing system to an optimum tuning frequency.
It is another objective of the present invention to manually move a tuning frequency to an optimum frequency according to a user""s demand after automatic fine tuning or channel switching.
In order to achieve the above objectives, the present invention is characterized in that an optimum frequency is determined referring to the SNR of a signal demodulated under the condition in which automatic fine tuning referring to an AFT voltage used in the conventional analog television set is not available.
In addition, the present invention is characterized in that there is provided an effective user interface, in which an optimum frequency position is determined by displaying a level corresponding to a SNR, rather than by observing a display state, in case of manual fine tuning.
In order to achieve the above objectives, there is provided an automatic fine tuning method for a digital television set in accordance with the present invention, including; a first step of determining whether or not a segment synchronization signal is detected after outputting tuning data of a corresponding channel during channel switching; a second step of setting a flag that has acquired synchronization when a segment synchronization signal is detected in the above step, storing the SNR of a corresponding frequency, moving the frequency, outputting tuning data of the moved frequency, and determining again whether or not a segment synchronization signal is detected; a third step of determining whether or not there is a segment synchronization signal acquired if there is not segment synchronization signal detected in the first step; a fourth step of tuning in to the largest SNR among stored SNRs if there is a segment synchronization signal acquired in the third step; and a fifth step of repeatedly performing the step for moving the frequency, outputting tuning data corresponding to the moved frequency and detecting a segment synchronization signal, if there is no segment synchronization acquired in the second step.
In addition, a manual fine tuning method for a digital television set in accordance with the present invention includes: a first step of displaying a signal level display pattern and fine tuning value display pattern on a screen of a television set; a second step of increasing or decreasing the displayed fine tuning value by inputting a fine tuning key, outputting tuning data corresponding to the position increased or decreased and adjusting the size of the signal level display pattern by detecting the signal to noise ratio; and a third step of inputting a confirm key when the size of the signal level display pattern is the maximum, judging the present frequency position to be the optimum, storing the SNR and finishing the manual fine tuning process.